In the preparation of magnetic recording materials, such as for magnetic disks and tapes, it has been common to use magnetic particles, such as gamma Fe.sub.2 O.sub.3, dispersed in a binder mixture to form the magnetic recording material. A dispersion is usually formed by milling the ingredients together for an extended period of time in an effort to thoroughly coat the magnetic particles with the binder ingredients and to break up collections or aggregations of such particles. Magnetic particles of this type tend to cling together and it is desirable to reduce or eliminate this aggregation of particles in order to produce smaller effective magnetic particle sizes for higher density magnetic recording. The degree of uniform dispersion of the magnetic particles in the binder is an important factor in determining the final quality of the magnetic coating, as measured by the parameters of surface smoothness, orientation ratio, signal-to-noise ratio, off-track performance, modulation noise, coercive force and wear properties.
After filtering, this dispersion is coated onto a substrate by spin coating, dip coating, spray coating or doctor blade (gravure) coating. The magnetic particles in the wet coating are then magnetically aligned by moving the coated substrate past a magnetic field. The coating is then cured to improve hardness and durability. The cured magnetic coating may be ground to reduce the coating thickness and then buffed to decrease surface roughness.
In order to increase the recording areal density, the coating thickness has to be reduced. Mechanical grinding to less than 5000 .ANG.is difficult, if not beyond the limit of present technology. Also, as the coating thickness is reduced, the signal amplitude is decreased and this presents a major problem. New, higher moment particles and a higher pigment volume concentration (PVC) are needed. PVC may be defined as the percentage of magnetic material in the coating relative to the total volume of the binder material and the volume of the magnetic material. The PVC of current magnetic coatings is about 20-30%, and it is difficult to increase this and still maintain the rheological properties of the magnetic particle dispersion.